The operation principle of semiconductor radiation detectors is based on a reverse biased pn-junction, creating a so-called depleted semiconductor volume, where an electric field is present. An incident photon (or a particle, such as alfa or beta particle or proton) causes a photoelectric effect, locally creating electron/hole pairs. The electric field of the depleted region segregates the charge carriers, one type of which is used as the signal charge. The measured amount of signal charge is used to determine the intensity of the radiation.
A known semiconductor radiation detector is the CCD (charge-coupled device), which can also be characterised as a charge transfer device (CTD), meaning that charge is transferred possibly long distances before it is measured. The early CCDs were of surface channel type devices, meaning that charge is transported at the silicon silicon-dioxide interface. The interface has, however, plenty of surface defects trapping the charge to be transported, thus decreasing the charge transport efficiency. A major improvement to the performance of CCDs was the transition to buried channel CCDs, where the signal charge is transported in a channel below the surface.
In front illuminated devices, where incident radiation comes through the charge transfer gates (usually made of polysilicon), the gate and isolation materials absorb a part of the radiation. The absorption is particularly intense for blue light, ultra violet (UV) and soft X-ray radiation and for low energy particles, impairing the so-called blue response of a radiation detector. An obvious way to improve the blue response is to use back illuminated devices, where all charge manipulating circuitry, i.e. thick material layers insensitive to radiation, are on the front side of the device.
The neutral substrate at the backside of traditional back illuminated CCDs must be etched away in order to obtain a good blue response, making these devices very thin: typically around 50 μm or less. The thinning process is difficult and likely to result in low manufacturing yield. The thin substrate causes also other problems. The penetration depth of red and near infrared photons in silicon is easily greater than the substrate thickness, resulting in bad red response and fringing, i.e. wavelike patterns in an image. The introduction of a thin biased backside layer, described e.g. in U.S. Pat. No. 6,025,585 and U.S. Pat. No. 6,259,085, combined with a high resistivity substrate, enabled the use of thick fully depleted substrates in back illuminated CCDs resulting both good red and blue response.
Blooming is an interfering effect that takes place when a bright spot in an image results enough of signal charge to fill the charge collection well of the corresponding pixel and starts to fill neighbouring pixels. This phenomenon can be prevented by the use of antiblooming structures. The fully depleted back illuminated CCD presented in U.S. Pat. No. 6,259,085 lacks, however, such antiblooming structures. Smearing is another problem observed during the charge transportation phase when a bright spot adds charge to all charge packets transported through it.
One additional problem in U.S. Pat. No. 6,259,085 and generally in CCDs is that the whole image frame has to be transported and read, even if only a fraction of the image would be of interest, making the operation of CCDs inflexible and slow. These problems are not present in active pixel sensors (APS), where the pixels can be read randomly and the signal charge is not transported, making them fast, flexible and immune to smear. Defective pixels on APS detectors will not affect other pixels unlike in CCDs, which increases fabrication yield and lowers production cost. The image quality can, however, be poor unless a high quality amplifier is attached in every pixel. The best way to accomplish the amplifier is to use the collected charge as an internal gate of a unipolar transistor like junction field effect transistor (JFET) or metal oxide semiconductor FET (MOSFET). From these transistors JFET is favoured. The internal gate structure consists of a potential energy minimum for the signal charges underneath the channel of a FET. The signal charges gathered in the potential energy minimum widen the channel, thus decreasing the channel resistance. The good amplifier properties of an internal gate FET are related to its small total capacitance to its small parasitic capacitance to total capacitance ratio and to the non-destructive reading allowing the signal charge to be read many times.
A good example of an internal gate structure is presented in U.S. Pat. No. 5,712,498 (where the internal gate is called the gate and the actual gate is called the back gate). In this patent a JFET structure is presented upon a buried channel forming the internal gate. The JFET source and drain areas are additionally oxide isolated from the semiconductor wafer. This amplifier structure is preferably used as an APS device but can equally well be used in a CCD structure. The device is back illuminated and must be thinned in order to achieve a good blue response. The red response is bad due to the thin nature of the device. Another prior art U.S. Pat. No. 5,786,609 presents a back illuminated APS radiation detector having a JFET equipped with an internal gate structure and a thick fully depleted substrate. The device has thus both a good red and blue response. In addition to that it has 100% fill factor.
The ultimate performance limit for semiconductor radiation detectors is set by the leakage or dark current, which mixes with the signal charge distorting the signal measurement. The leakage current can be divided to three components. One component arises from depleted regions in the device. Since operation of semiconductor detectors is based on the depletion region, this current component cannot be eliminated. Reducing the depletion region size decreases this current component, but on the other hand this degrades the sensitivity for deeply penetrating radiation. The only reasonable way to minimise this current component is to minimise the amount of defects in the semiconducting material, i.e. one should use high quality substrates and carefully selected manufacturing processes.
A second leakage current component is the diffusion current arising from depletion region boundaries. This component is, however, only significant at depletion regions borders in high resistance material. In fully depleted detectors made of highly resistive material this is the case only outside the active area, i.e. outside the area where the pixels are situated. This current component can be easily eliminated for instance by surrounding the active area with a biased guard ring.
The third and usually most prominent source of leakage current is the interface current, also known as the surface generation current. This current component arises from depleted areas in the semiconductor next to the semiconductor surface or interfaces with different materials, and is referred to later on in the text as surface current.
The reason why surface current forms such a big portion of the total leakage current is due to the fact that the density of defects is high at surfaces and interfaces. Silicon has been widely used as a detector material since high quality substrates are easily available and since the silicon to silicon dioxide interface has a relatively low amount of defects. Even in silicon based detector structures the surface current is usually a major source of leakage current. In U.S. Pat. No. 6,259,085 for instance surface current is the main source of leakage, although the device is operated in multi pinned phase (MPP) state during the charge integration phase. The MPP mode is used to eliminate the surface current during the charge integration period, but it cannot be used during the charge transport period. The structure in U.S. Pat. No. 6,259,085 illustrates well the problematic nature of leakage current; it has a thick fully depleted silicon substrate, MPP operation is used during charge integration period, and surface current is still the major source of leakage.
A well-known way for decreasing leakage current is efficient cooling. However, this requires either complicated liquid gas cooling arrangements or power-intensive peltier element cooling, neither of which is particularly attractive for use e.g. in portable appliances, where both complicatedness and power consumption should be kept at minimum.
Weaknesses of the structure in U.S. Pat. No. 5,712,498 come from the fact that there is no antiblooming structure and that the surface generated charges are not separated from the signal charges. The structure of the later versions of this DEPFET device suggest, however, that the surface generated charges can be collected by a clear contact (marked as L in the patent). Even though the internal gate structure has severe limitations. First of all an exeptionally good homogeneity of the internal gate doping (marked as 1 in the patent) is required. Secondly the use of MOSFET in conjunction with the internal gate structure is problematic since the MOSFET channel has to be always open to prevent surface charges from mixing with the signal charges situated in the internal gate structure. Thirdly the use of a Bipolar transistor in conjunction with the internal gate structure is not possible.